In order to transmit large-volume data by the use of limited frequencies, multiple-input and multiple-output (MIMO) systems for performing spatial multiplex transmission using a plurality of transmitting and receiving antennas are being developed. The number of spatial multiplexing is expected to continue to increase for further improvements in frequency utilization efficiency.
In the next-generation of mobile communication systems, the use of high frequencies exceeding 6 GHz is being studied. However, the use of a high-frequency band disadvantageously increases propagation loss. On the other hand, for the use of a high-frequency band, antenna elements can be increased to form a transmission beam with high gain. Thus, it is being studied to compensate, by forming a transmission beam, for the disadvantage of propagation loss increase. A technology of combining MIMO transmission and an array antenna formed of a multi-element antenna is being developed to avoid a reduction in the cover area of a cell while achieving large-volume data transmission.
Patent Literature 1 describes an invention that combines MIMO transmission and a plurality of array antennas, and controls the antennas to form transmission beams that reduce correlation between MIMO streams and increase reception quality. The invention described in Patent Literature 1 sweeps transmission beams at small angular intervals when determining transmission beam shapes, thereby searching for a plurality of beams of low correlation and good reception quality.
Patent Literature 2 describes an invention that selects a combination of beam antennas so as to maximize the signal-to-interference-and-noise ratio (SINR) of each MIMO stream, and further minimize the correlation coefficient of a channel transfer matrix.